Heat shrinkable plastic products have a property of being shrunk when heated and so are widely used as films for a shrinkage package, a shrinkage label, or the like. Among them, polyvinyl chloride (PVC), polystyrene, and polyester-based plastic films have been used as labels or cap seals of various containers or used as a direct package material, or the like,
However, films made of polyvinyl chloride are subject to environmental restrictions since at the time of incinerating the film, hydrogen chloride gas and a dioxin-causing material may be generated. In addition, when this product is used as a shrinkage label of a polyethylene terephthalate (PET) container, or the like, at the time of reusing the container, a troublesome process of separating the label and the container from each other has to be undergone.
Further, the polystyrene-based films have good working stability when undergoing the shrinkage process, and the appearance of the product may be good, but have poor chemical resistance, undesirably requiring that an ink having a specific composition should be used for printing. Furthermore, the polystyrene-based the films have a disadvantage in that since storage stability at room temperature is insufficient, the film may become spontaneously shrunk, undesirably deforming the dimensions thereof.
In order to solve the above-mentioned problems, a film made of a polyester resin has been studied and developed as a film capable of replacing the films made of the above-mentioned two raw materials. Moreover, as a use amount of the PET container increases, a use amount of a polyester film capable of being easily reused without separately separating a label at the time of reuse has been gradually increased, but a heat shrinkable polyester film according to the related prior art had a problem in view of shrinkage characteristic. That is, there was a problem in that wrinkles at the time of shrinkage or non-uniform shrinkage during a molding process were frequently generated due to a rapid change in shrinkage behavior. In addition, a shrinkage property of the polyester film at a low temperature was decreased as compared to the polyvinyl chloride-based film or the polystyrene-based film. In order to complement this disadvantage, therefore, the polyester film should be shrunk at a high temperature. In this case, there were problems in that PET container may be deformed, or a white-turbidity phenomenon may occur.
Thus, there has been continuous demand to develop a copolymerized polyester resin having an excellent shrinkage rate and an improved shrinkage property at a low temperature, and a heat shrinkable film comprising the polyester resin above.
On the other hand, if it is desired to obtain a molded product in the form of a film as a heat shrinkable film using the polyester resin, it is necessary to further improve the adhesion property of such molded product or its intermediate, a molten sheet. The reason is that, in order to produce a film with a thinner and uniform thickness at high speed using the polyester resin, the molten sheet extruded from the extruder needs to be in close contact with the cooling means (for example, cooling roll) and solidified in the course of preparing the molded product in the form of a film. Furthermore, recently, as the thickness of the film to be produced becomes thinner, such demand has been increased.
For this reason, in the past, a method of adding an electrostatic applying agent to the molten sheet to improve the application and adhesion properties was applied. In this case, however, the insulating properties of the polyester resin and the heat-shrinkable film comprising the polyester resin can be significantly deteriorated.
To solve these problems, methods for improving the adhesion property by closely adhering to the molten sheet by the power of wind using an air knife instead of an electorostatic applying agent or by spraying or coating a liquid having a low surface tension with a rotating cooling roll and the like have been suggested. However, these methods have disadvantages in that the whole process is complicated and difficult to control.